In process automation as well as in manufacturing automation, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are measuring devices, such as, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-measuring devices, conductivity measuring devices, etc., which register corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity, respectively. Used for influencing process variables are actuators, such as valves or pumps, via which e.g. the flow of a liquid in a pipeline or the fill level of a medium in a container is changed. The terminology ‘field devices’ in connection with the invention is, thus, intended to include all types of measuring devices and actuators.
Moreover, the terminology ‘field devices’ in connection with the invention also includes all devices applied near to the process and delivering or processing information relevant to the process. Besides the above mentioned measuring devices/sensors and actuators, also referred to as field devices are generally any units, which are connected directly to a fieldbus and serve for communication with the superordinated unit, thus units such as e.g. remote I/Os, gateways, linking devices and wireless adapters, respectively radio adapters.
A large number of such field devices are produced and sold by the Endress+Hauser group of firms.
In modern industrial plants, communication between at least one superordinated control unit and the field devices occurs, as a rule, via a bus system, such as, for example, Profibus® PA, Foundation Fieldbus® or HART®. Nowadays, the use of standard Ethernet protocols in an industrial environment, as for example in automation or process control, is becoming more and more frequent. For using the Ethernet protocols Ethernet-enabled field devices have been developed, which allow the measured data to be communicated over networks such as Ethernet-based LANs (Local Area Network) or the Internet, so that the data can be monitored and managed online from an also Ethernet-enabled device connected to the Ethernet LAN or the Internet.
Today, the Ethernet-enabled field devices need two independent cables: one for supplying power, and another cable for communicating the data over the Ethernet. However, the installation of the second cable for supplying power can be difficult sometimes and further results in major costs.
Consequently, a technology called Power over Ethernet (hereinafter PoE) has been developed to transmit power over an Ethernet cable to an Ethernet-enabled device. This approach eliminates the need for two independent cables to connect a device to another as well as to deliver power to such a device.
Recently, an IEEE standard has been developed that addresses PoE issues, namely the standard IEEE802.3af, which defines the specification for Ethernet power sourcing equipment and powered devices. The IEEE802.3af standard specs the voltage on the cable, the current on the cable as well as the power on the PoE receiving device. This specification standardized on the use of 48 volts of direct current as the injected PoE voltage.
This technique is limited by DC power losses in the cabling, and complicates galvanic isolation and other safety measures in the field of automation technology.